1) Field of the Invention
The present invention relates to a technology for fixing a toner image on a recording medium by applying heat and pressure to the toner on the toner image during passage of the recording medium through a nip portion between a fixing belt that rotates and a facing member provided so as to face the fixing belt.
2) Description of the Related Art
An image forming apparatus is constituted as a digital copier, a printer, a facsimile, or a multifunction machine having at least two functions of these devices. The image forming apparatus conventionally adopts a fixing apparatus of the type described above. In this type of fixing apparatus, the heat capacity of the fixing belt used for the fixing apparatus can be made relatively small. Therefore, when the fixing apparatus is started up, it is possible to reduce a time required for increasing the temperature of the fixing belt to a fixing temperature suitable for fixing the toner image.
This type of fixing apparatus can be classified roughly into those as follows. That is, (1) a first type of fixing apparatus having first to third heaters that heat a heating member around which an endless fixing belt is wound, a support member, and the facing member, respectively, (2) a second type of fixing apparatus having one heater disposed inside a heating member, and the other heater(s) disposed inside either one of a support member and a facing member, or disposed inside both of the support member and the facing member, and (3) a third fixing apparatus having one heater that heats a fixing belt, and the other heater(s) disposed inside either one of a support member and a facing member, or disposed inside both of the support member and the facing member.
Conventionally, in any type of the fixing apparatuses, the rate of power supplied to the respective heaters is always constant. With such a configuration, for example, a defect described below may come up.
In the first type of fixing apparatus, the first to third heaters are energized and supplied with power at a certain rate at the time of fixing operation, and are controlled so that the temperature of the fixing belt and the facing member is increased to a fixing temperature suitable for fixation of the toner image. Further, at the time of startup and during a standby state of the fixing apparatus, power is supplied to the first to third heaters at a certain rate. However, in such a control mode, when a recording medium passes through the nip portion between the fixing belt and the facing member during the fixing operation, much of the heat of the fixing belt is taken away by the recording medium, thereby the temperature of the fixing belt rapidly decreases. Therefore, even if the fixing belt is heated by the first to third heaters, the temperature of the fixing belt is not restored in time and becomes lower than the fixing temperature suitable for fixing the toner image, thereby insufficient fixing may be caused. Such a defect similarly occurs in the second and third types of fixing apparatus.